This invention relates to cathodes for batteries.
Batteries are commonly used electrical energy sources. A battery contains a negative electrode, typically called the anode, and a positive electrode, typically called the cathode. The anode contains an active material that can be oxidized; the cathode contains or consumes an active material that can be reduced. The anode active material is capable of reducing the cathode active material. In order to prevent direct reaction of the anode material and the cathode material, the anode and the cathode are electrically isolated from each other by a separator.
When a battery is used as an electrical energy source in a device, such as a cellular telephone, electrical contact is made to the anode and the cathode, allowing electrons to flow through the device and permitting the respective oxidation and reduction reactions to occur to provide electrical power. An electrolyte in contact with the anode and the cathode contains ions that flow through the separator between the electrodes to maintain charge balance throughout the battery during discharge.
In a metal-air electrochemical cell, oxygen is reduced to hydroxide ions at the cathode, and a metal, e.g., zinc, is oxidized at the anode. Oxygen is supplied to the cathode from the atmospheric air external to the cell through one or more air access opening(s) in the cell container.
A cathode material used for oxygen reduction in zinc-air cells includes a metal oxide catalyst (e.g., manganese dioxide) dispersed on carbon (a support). It is believed that the carbon reduces oxygen (O2) to peroxide (O2Hxe2x88x92), while the catalyst decomposes peroxide (O2Hxe2x88x92) to oxygen (O2) and hydroxide (OHxe2x88x92). Because the two-step reduction process involves diffusion of peroxide from the carbon to the catalyst, it is generally desirable for the catalyst to be dispersed highly and evenly on the carbon. For example, a low dispersion of the oxide on carbon can lower the discharge rate of the cathode. Moreover, the buildup of peroxide can decrease the running voltage and limiting current of the cathode.
The invention relates to cathode active materials formed from metal naphthenate precursors, such as cobalt naphthenate. A catalyst such as cobalt and/or cobalt oxide is believed to be capable of reducing oxygen to hydroxide without formation of peroxide, as well as reducing peroxide to oxygen and hydroxide. The precursors provide a cost-effective method of forming highly-dispersed, finely-divided metal/metal oxide on a support, e.g., carbon. Furthermore, when incorporated into a battery, the cathode active materials generally provide the battery with high limiting current and high running voltage.
In one aspect, the invention features a method of making a cathode including decomposing a mixture comprising a metal naphthenate and incorporating the decomposed mixture into the cathode. The mixture can further include carbon and a non-aqueous solvent, e.g., acetone or alkanes.
Embodiments of the invention may include one or more of the following features. Decomposing the mixture can be performed by evaporating the non-aqueous solvent, heating the mixture to about 50xc2x0 C. to about 70xc2x0 C., and/or contacting the metal naphthenate with a chemical reducing agent. Incorporating the decomposed mixture into the cathode may include incorporating carbon and a binder, e.g., polytetrafluoroethylene, into the cathode.
In another aspect, the invention features a method of making a battery including forming a solution comprising a metal naphthenate and carbon in a solvent, removing the solvent, and decomposing the metal naphthenate to form a metal oxide dispersed on the carbon. The decomposed naphthenate can be incorporated into a cathode, which can be incorporated into a metal-air battery or into an electrode of a fuel cell.
In another aspect, the invention features a method of making a cathode including decomposing a metal naphthenate and incorporating the decomposed naphthenate into the cathode. The metal naphthenate can include cobalt naphthenate.
Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.